I still saw this with cs:trusty/rabbitmq-server-48

juju status was showing this about rabbit:
rabbitmq-server/0                   active         idle        1.25.5  2/lxc/0 5672/tcp                                10.96.6.63        Unit is ready and clustered                                      
rabbitmq-server/1                   maintenance    executing   1.25.5  1/lxc/2 5672/tcp                                10.96.8.95        Starting RabbitMQ                                                
rabbitmq-server/2                   active         idle        1.25.5  5/lxc/0 5672/tcp                                10.96.9.186       Unit is ready and clustered                            

and the logs on rabbitmq-server/1 were full of:
Mnesia('rabbit@juju-machine-1-lxc-2'): ** ERROR ** Mnesia on 'rabbit@juju-machine-1-lxc-2' could not connect to node(s) ['rabbit@juju-machine-5-lxc-0']

/etc/hosts on rabbitmq-server/1 had:
# cat /etc/hosts
127.0.0.1 localhost

# The following lines are desirable for IPv6 capable hosts
::1 ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
ff02::3 ip6-allhosts
10.96.8.95 juju-machine-1-lxc-2
10.96.6.63 juju-machine-2-lxc-0


Process listing was:
root         922  0.0  0.1 511780 22576 ?        Ssl  16:07   0:00 /var/lib/juju/tools/machine-1-lxc-2/jujud machine --data-dir /var/lib/juju --machine-id 1/lxc/2 --debug
root        1025  0.0  0.1 487008 22728 ?        Ssl  16:20   0:00 /var/lib/juju/tools/unit-rabbitmq-server-1/jujud unit --data-dir /var/lib/juju --unit-name rabbitmq-server/1 --debug
root       10406  0.0  0.1  92464 20136 ?        S    16:26   0:02  \_ /usr/bin/python /var/lib/juju/agents/unit-rabbitmq-server-1/charm/hooks/cluster-relation-changed
root       11222  0.0  0.0   4440   668 ?        S    16:27   0:00      \_ /bin/sh /usr/sbin/rabbitmqctl start_app
root       11230  0.0  0.0  61672  1676 ?        S    16:27   0:00          \_ su rabbitmq -s /bin/sh -c /usr/lib/rabbitmq/bin/rabbitmqctl  "start_app"
rabbitmq   11231  0.0  0.0   4440   644 ?        Ss   16:27   0:00              \_ sh -c /usr/lib/rabbitmq/bin/rabbitmqctl  "start_app"
rabbitmq   11232  0.0  0.1 370876 24084 ?        Sl   16:27   0:00                  \_ /usr/lib/erlang/erts-5.10.4/bin/beam.smp -- -root /usr/lib/erlang -progname erl -- -home /var/lib/rabbitmq -- -pa /usr/lib/rabbitm
rabbitmq   11264  0.0  0.0   7456   428 ?        Ss   16:27   0:00                      \_ inet_gethost 4
rabbitmq   11265  0.0  0.0   9548   632 ?        S    16:27   0:00                          \_ inet_gethost 4
rabbitmq    8002  0.0  0.0   7492   320 ?        S    16:24   0:00 /usr/lib/erlang/erts-5.10.4/bin/epmd -daemon
rabbitmq   10803  0.0  0.0   4440   648 ?        S    16:26   0:00 /bin/sh /usr/sbin/rabbitmq-server
rabbitmq   10834 19.2  0.3 2174496 55720 ?       Sl   16:26   9:16  \_ /usr/lib/erlang/erts-5.10.4/bin/beam.smp -W w -K true -A30 -P 1048576 -- -root /usr/lib/erlang -progname erl -- -home /var/lib/rabbitmq -- -pa /us
rabbitmq   10943  1.6  0.0   7456   428 ?        Ss   16:26   0:46      \_ inet_gethost 4
rabbitmq   10944  5.2  0.0  13776   784 ?        S    16:26   2:32          \_ inet_gethost 4


I added this to /etc/hosts:
# added manually by andreas
10.96.9.186 juju-machine-5-lxc-0

And just by saving that file, rabbitmq-server/1 unwedged and the deployment proceeded.